I think that, except for special, 'call' gigs, you have to understand that your employers (the clubs and restaurants, at least) are under similar pressures as you. Higher gas costs mean fewer people are driving as far to go eating and dancing. Which means less money for them, and asking more at that particular time might be difficult (if not suicidal!).

The time to ask for more money is when things are going well, not when things start to get tight!